To speak very plainly, yes, everyone is technically a starseed. This is the case because everyone has had lives beyond earth. Our earth is a dense third dimensional planet that is the conglomerate of the karma of many, many higher dimensional planets and stars. In order to begin incarnating here you have to move down the higher dimensions and arrive on earth.
Earth is a very difficult planet in a dimension of the cosmos that forces us to reconcile the parts of us that do not understand our light. We have the opportunity to consolidate huge amounts of karma here which translates to experiencing lots of division, contrast and polarity. It is not a planet, or density/dimension, that one begins on or arrives at with no experience. An immature soul would not be able to handle the fragmentation process, let alone have enough experiences to fragment into the third dimension. So, part of this question isn’t necessarily about an individual souls identification, but rather the incarnation structure of the earth itself.
People have stated that they are new souls or souls that have never incarnated anywhere else. In my opinion, that is an oversight, and the individual is not accessing their souls cosmic history. We can choose (consciously or unconsciously) to either become aware of our souls path, or we can choose to be unaware. Some people will choose to not recognize their souls experiences in certain areas of the cosmos, or even certain eras on the planet. We do this to create a specific perspective and experience. This is natural and helpful to us, but our perspective can always evolve!
Now that that question is addressed on the most fundamental level, we can begin to discuss why people have such unique experiences in regards to their soul lineage. Why do some people identify as a starseed and have cosmic experiences while others don’t? Well, Cosmic experiences are specific spiritual realizations, they change you. They change how you view the world and live your life. To speak plainly again, not everyone is looking to get that deep. Further, you don’t have to be aware of your cosmic connections to live a productive and meaningful life. This means that you have some people that are aware, and growing in that awareness, and then you have others that are unaware and yet also growing.
It is our choice what pieces of our soul we wish to recognize in our life and how deeply we choose to change in response to these realizations, how much we wish to wake up. What is not often considered when we discuss the awakening process is the importance of the pace in which we awaken. It is a challenge to find a balance between remembrance and growth and rebirth and destruction. Every time that we remember something about our soul, a part of our selves rooted in fear must die. The light of our spirit that we block, we block because we fear it. The reason that we are not embodying our light fully is because we have beliefs that prevent us from doing so. It takes time to remove these beliefs, as simply trying to disregard all fear without deep awareness can be incredibly destabilizing. We are designed to respectfully observe our fears and see how they react in the presence of light, of self love, gentleness and curiosity. The more we do this the more fluent and fast we become at spiritual growth. We begin recognizing patterns and craving to know our soul and its path on the deepest level possible. Not all people on this planet wish to jump into spiritual exploration, let alone the deepest and most challenging variety of spiritual exploration. Over time humanity will go in this direction, the collective consciousness will evolve to include other worlds, however for now, we are simply not there.